Yes, there's a minimum flash duration but I think the camera and flash
circuits are faster than you think. I have a little Vivitar 550FD which
does TTL on Minolta, Pentax and Olympus. At ISO 100 it specs minimum
distance at 2.1 feet at f/5.6 or smaller, 3 feet at f/4 and 6 feet at
f/2. The T-32 specs 0.5 feet at f/22, 1.1 feet at f/11, 1.6 feet at
f/8, 2.2 feet at f/5.6 and (a bit slower than the lowly 550FD) 6.5 feet
at f/2.
Using auto mode rather than TTL gives approximately the same minimum
distances. The 550FD only has options for f/2 and f/4 giving minimum
distances of 6 and 3 feet respectively. The T-32's auto mode options of
f/4, f/5.6 and f/8 give minimum distances of 3.2 feet, 2.3 feet and 1.7
feet.
Real comparison of these flash units for minimum distance is difficult
since the 550FD puts out much less light than the T-32 which means it
has to be used at larger apertures. I remember trying to use the 550FD
on the 5D one time. I don't recall the aperture but it was probably in
auto mode at f/2 and it failed to quench in time to prevent overexposure
from about 4 feet. Now I see why. But auto mode at f/4 should have worked.
Chuck Norcutt
On 5/17/2012 7:40 AM, Jez Cunningham wrote:
> As you say, the response time of the quench circuit in the flash unit
> will be measured in hundreds of microseconds, whereas the speed of
> light is orders of magnitude faster (and is thus irrelevant).
> That means that there's a minimum flash duration that may still be
> giving too much light for the subject (at the selected aperture)
> Jez
>
> On 17/05/2012, Chuck Norcutt<chucknorcutt@xxxxxxxxxxxxxxxx> wrote:
> But if you get too close with auto flash control the electronics may
> not have enough time
>> to respond. Remember one foot to the target and another foot back to
>> the TTL flash sensor is only about 2 nanoseconds. I have no idea what
>> the minimum distance spec is for the built-in on the E-510.
>>
>> Chuck Norcutt
>>
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